Electronic simulation for compensating laser diode thermal effects

Abstract: An electronic compensation circuit (24; Fig.4) in which a simulation of a laser's thermal droop characteristics is used to adjust input to the laser and thereby compensate for the laser's thermal droop effects The simulation circuit is connected to the laser either as an open loop circuit to provide a once only correction or as a closed loop feedback circuit to provide a continuous correction of the laser droop effects. Video data signal (20) on line (22) is connected to switch S that switches between laser control signal (36) on line (38) or AGC signal (30) on line (32) and ground (42). Thermal effects correction signal (26) is connected to the laser control signal (36) through an RC circuit (44,46) whose time constant is equal to the falling time constant t fs (Fig.1). As capacitor (46) charges, a small signal proportional to laser control signal (36) appears on line (28). This is the termal effects correction signal and satisfies I=Ae (-t/RC) , where A is an arbitrary constant. The simulation circuit is designed using either analog or digital circuitry and is adjustable to provide compensation for lasers having different time constants of droop or different values of droop.

Description: This invention relates generally to laser diodes and more particularly concerns an electronic simulation in which thermal effects are modeled and used for correcting laser diode output. A single beam laser diode assembly has a single diode and usually, in a scanning system, the diode is driven by a train of image pixel information. The pixel information is used to drive the diode and therefore stimulate laser flux emission where there is a white pixel in a write white system. In a write white system, a laser is turned on to create white space on a page. Intensity of the light beam is directly proportional to the output power of the laser. In order to keep the output power of the diode constant, the temperature of the diode should be kept at a constant level. However, due to the structure of the laser diode assembly, as the pixels change, the temperature of the diode fluctuates, which in turn causes the output power of the diode and the intensity of the light beam to fluctuate. A multiple beam diode assembly has at least two diodes in close proximity on a common substrate. Each diode is driven by a separate train of image pixel information. Again, as the pixels change, the temperature of each diode fluctuates. However, in a multiple diode system, the changing temperature of a diode also causes a temperature fluctuation in adjacent diodes. The temperature fluctuations of the adjacent diodes cause the output power and the intensity of the light beams in those adjacent diodes also to fluctuate. A tri-level system may use one or more diodes with at least one diode operating at full on, full off, and partially on. One example of an application using a single diode tri-level system is the printing of black and white documents with a highlight color. Tri-level systems suffer from the same heating effects both in the full on and the partially on modes of the laser. In a printing system fluctuation in the intensity of light beams causes fluctuation in the size of a printed pixel. As the intensity of the light beam decreases, so does the pixel size. In Figure 1, the intensity variation in a diode over time is shown due to heating and cooling effects. To illustrate the effect, a first laser beam 10 is left turned on while a second laser beam 12 from an adjacent laser is cycled from a full on position to a full off position using a step function. While the second laser beam 12 is on, heating effects on the first beam 10 cause the intensity to drift downward and finally stabilize at a lower value. The change in intensity is the drift d c . A similar curve is produced from self-heating effects in the second laser 12 when it is turned on. The self-heating effects are seen in the falling time constant t fs . The rising time constant t rc or the falling time constant t fc when compared to falling time constant t fs , produced from self-heating, is larger. When the laser beam 12 has settled after self-heating effects, the difference in the output intensity is the drift d s When the drift d c is compared to the drift d s , the drift d c is smaller than the drift d s . When the second laser beam 12 is turned off the heat dissipates. As the heat dissipates, the intensity of the first beam 10 drifts upward and stabilizes at a higher value. The amount the intensity changes is the intensity drift d c . The amount of time needed for the intensity to drift and stabilize is a rising time constant t rc . When measured practically, rising time constant t rc and falling time constant t fc are nearly the same. This is important in designing a simulator since both the rising time constant t rc and falling time constant t fc can then be adequately modeled using a single circuit for both. A more accurate but more complicated circuit could be built to model the rising time constant t rc and the falling time constant t fc independently of each other. Large drifts are visible in printed pages. In Figure 2, an enlargement of a half tone pattern that illustrates the problem is shown. The pattern consists of six scan lines s1-s6 of alternating light and dark areas. For simplicity, it will be assumed that a single diode assembly in a write white system was used to scan each of the six scan lines s1-s6 sequentially starting with scan line s1 and progressing through scan line s6. However, similar problems and effects occur in write black systems, multiple diode assemblies and tri-level systems. Figure 2 shows the half tone pattern when it is correctly printed with no heating effects. Scan lines s1 and s5 are all black. Scan lines s2, s3, and s6 are alternating blocks of black and white. Scan line s4 is all white. In a write white system, the laser is turned on to create white space and turned off to create black space. Rectangle R1 is the first white rectangle printed on scan line s2. Figure 3 is the pattern shown in Figure 2 when it is printed with a single laser diode experiencing self-heating effects. When scanning scan line s1 a laser remains off the entire time since the entire line is black. When scanning scan line s2, the laser starts scanning in the off position but shortly turns on when it enters rectangle R1. When the laser turns on, the spot intensity is at its peak and gradually diminishes and stabilizes as shown in Figure 1. The resulting change in spot size from large to small will create sloping edges E on rectangle R1. Every time the laser turns on. the same sloping edges E will be produced until the intensity and spot size stabilize. Similar effects can be seen in multiple laser assemblies, tri-level systems and write black systems. In multiple laser assemblies, the deviations are caused from heating effects from adjacent lasers as well as the writing laser. In tri-level systems heating effects occur at both the full on and the intermediate on states. Minimizing heating effects or otherwise compensating for or correcting for them would result in more accurate printed pages with improved image quality. WO-A-93/18563 discloses a method and apparatus for maintaining a controlled precise energy output level for a laser diode (12), particularly useful in a precision scanning environment, which provide for a thermal stabilization current to be added to the normal bias exposure current of the diode (12). The thermal stabilization current is modeled to account for the temperature variation of the laser diode junction which affects the light energy output as a function of current. The addition of the thermal stabilization current provides a substantially flat and constant, precise output energy from the laser diode (12) during its "on" time as a function of an input exposure value. JP-A-60-64853 discloses a drive system for a laser diode, in which a drive circuit has a correcting current source for dealing with thermal time constant. A transistor three resistors (RE, R1, R2) and a capacitor (C1) form the correcting current source. The thermal time constant of the junction of the laser diode is equalised to the time constant of R1, R2 and C1. One method for compensating for laser drift would be to use a direct, real time feedback system. However, direct feedback systems have the disadvantages of being very expensive to construct, requiring extremely fast components to effectively calculate and provide a real time corrective signal, and requiring additional light paths in the printing system. Therefore, some other method must be found. The present invention uses the idea that laser drift can be modeled accurately enough to generate an appropriate correction signal. When the pattern dependent laser drive signal is presented to both the laser diode and its model then the laser drift can be computed using the model. The correction of the laser drift can then be implemented and presented to the model to observe the correction effects. The important requirement is to make a model which faithfully models the physics of the actual heating effects of the laser. A compensation method based on a simulation of the laser heating effects has the advantages of being inexpensive and reliable while requiring no additional light paths and being easy to install and adapt to many different printing systems. Accordingly, it is the primary aim of the invention to provide a method for compensating for a variety of thermally induced effects. Further advantages of the invention will become apparent as the following description proceeds. The present invention provides a laser with intensity control comprising: a) a laser, b) simulator means for simulating laser thermal effects and generating a compensating signal in accordance with the simlated laser thermal effects, the simulator comprising: i) a control means for receiving a control signal; ii) an input means for receiving an input signal; iii) a current generating means for generating said compensating signal said current generating means being operably connected to said input means and to said control means, and being responsive to said control means to generate said compensation signal only when said control means has received a predetermined control signal, said current generating means being operably connected to said control means to generate the increasing current signal when said control means has received the control signal; and iv) output means operably connected to and responsive to said current generating means for transmitting the compensating signal, v) said compensating signal being approximately described by the equation I=Ae (-t/RC) +B wherein: A = a first constant, and B = a second constant corresponding to an offset voltage, e = the base for natural logarithms approximately equal to 2.71828, and the elements in the exponent: t = time constant of the circuit, R = resistance of the circuit, and C = capacitance of the circuit, c) means providing a base level gain control signal for driving said laser, d) adding means operatively connected for receiving base level gain control signal and the compensating signal for combining said base level gain control signal and the compensating signal to generate a laser driving signal for controlling the intensity of the laser, and e) said laser being operatively connected for receiving the laser driving signal. The invention further provides a laser system according to claim 9 of the appended claims. Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a graph of a laser ouput intensity as it varies due to laser drift; Figure 2 is an enlargement of a pixel pattern; Figure 3 is an enlargement of a pixel pattern affected by laser drift; Figure 4 is a block diagram of a laser compensating circuit for a single laser; Figure 5 is a block diagram of a laser compensating circuit for a single laser; Figure 6 is a schematic of a laser drift simulation circuit before activation; Figure 7 is a schematic of a laser drift simulation circuit after activation; Figure 8 is a graph of the output of the laser drift simulation circuit shown in Figure 7; Figure 9 is a graph of the output of a digital laser drift simulation; Figure 10 is a schematic of an adjustable laser drift simulation circuit; Figure 11 is a schematic of an adjustable laser drift simulation circuit; Figure 12 is a schematic of an adjustable laser drift simulation circuit; Figure 13 is a block diagram of a laser compensation circuit for a dual laser system; Figure 14 is a schematic of an adjustable laser drift simulation circuit used in Figure 13; and Figure 15 is a schematic of an adjustable laser drift simulation circuit used in Figure 13. Turning now to Figure 4, video data input signal 20 on line 22 is applied to a thermal effects simulator 24. Also applied to the thermal effects simulator 24 is the automatic gain control signal 30 on line 32. The automatic gain control signal 30 adjusts to control the light laser 40 due to long term effects such as aging of the photoreceptor. Next, the video data input signal 20 is used to activate a switch which turns a laser 40 on or off according to the desired printed pattern. In the write white example, the video data input signal 20 becomes positive corresponding to white space on a printed page and the laser 40 is turned on . The thermal effects simulator 24 responds to the video data input signal 20 and the automatic gain control signal 30 by outputting a signal linearly proportional to the automatic gain control signal 30 when modulated by the effects of simulated temperature rise. The new signal transmitted by the thermal effects simulator 24 is the thermal effects correction signal 26 on line 28 and is used to compensate for thermal droop effects. The thermal effects correction signal 26 and the automatic gain control signal 30 on line 32 are both applied to an adder 34. The automatic gain control signal 30 is determined by photoreceptor sensitivity and other system factors. Since, over time, the photoreceptor becomes less sensitive, the signal must be increased to increase the laser 40 output. A laser control signal 36 on line 38 is emitted by an adder 34 to be applied to laser 40. The laser control signal 36 contains both the thermal effects correction signal 26, to correct for thermal effects, and the automatic gain control signal 30, to correct for photoreceptor aging effects. The thermal effects simulator 24 contains circuitry which appropriately simulates laser droop and generates a correction signal. The simlator is programmed or implemented with a standard time constant. Using the falling time constant t fs , the thermal effects simulator 24 can compensate for the drift d s in intensity (as shown in Figure 1) by increasing the thermal effects correction signal 26. The increased thermal effects correction signal 26 will cause an increase in intensity of the laser. The circuit in Figure 4 applies a first order corrective increase in the current to compensate for a downward drift in laser intensity. The laser will respond to the increased current by increasing its output, which will also generate more heat causing additional thermal effects. A circuit shown in Figure 5, can be used to compensate for higher order thermal effects. In the circuit shown in Figure 5, the laser control signal 36 is applied to the thermal effects simulator 24 rather than the automatic gain control signal 30. When the video data input signal 20 is first turned on the laser control signal 36 is equal to the automatic gain control signal 30 and the thermal effects correction signal 26 is equal to zero. As the thermal effects simulator 24 responds, the thermal effects correction signal 26 rises and is added to the automatic gain control signal 30 to form the laser control signal 36. As the laser control signal 36 rises, it is fed back to the thermal effects simulator 24 which can then increase the thermal effects correction signal 26 further to compensate for the additional thermal effects. In its simplest form, the circuitry inside the thermal effects simulator 24, shown in Figures 4-5, is shown in Figure 6. The video data input signal 20 on line 22 is connected directly to a two pole switch S that switches between the laser control signal 36 on line 38 or the automatic gain control signal 30 on line 32 and ground 42. In Figure 6, the two pole switch S is shown connected to ground 42. The thermal effects correction signal 26 and one side of a capacitor 46 are then connected to ground 42 through the resistor 44. The other side of the capacitor 46 is always connected to ground 48. The thermal effects correction signal 26 is zero since it is pulled to ground 42. Figure 7 shows the identical circuit with the two pole switch S connected to the laser control signal 36. This happens when the video data input signal 20 goes high (in our write white system). The thermal effects correction signal 26 is now connected to the laser control signal 36 through the RC circuit consisting of the resistor 44 and the capacitor 46. The values of the resistor 44 and the capacitor 46 are chosen to produce a time constant for the RC circuit that is equivalent to the falling time constant t fs shown in Figure 1. As the capacitor 46 charges, a small signal that is proportional to the laser control signal 36 on line 38, appears on line 28. This signal is the thermal effects correction signal 26 and is described by the equation I= Ae (-t/RC) where: A = an arbitrary constant, and e = the base for natural logarithms approximately equal to 2 71828, and the elements in the exponent: t = time constant of the circuit, R = resistance of the circuit, and C = capacitance of the circuit. Figure 8 shows this signal, on a graph over time. Referring to Figure 8, the thermal effects correction signal 26 starts at a value of zero, as the capacitor 46 charges, the signal slowly starts to rise and finally levels off. There are many ways, known in the art, to create the thermal effects correction signal 26 shown in Figure 8. The RC circuit could be replaced with an equivalent RL circuit, multiple RC or RL circuits, or any combination thereof, as known in the art. Alternatively, the signal could be approximated digitally, using a miroprocessor, ROM, RAM, or other digital circuit, as known in the art, as shown in Figure 9. For comparison, in Figure 9, the ideal thermal effects correction signal 26 is shown as a fine dotted line while the actual thermal effects correction signal 26 is shown as a solid line. This approach works well provided that the falling time constant t fs (shown in Figure 1) and the drift d s (shown in Figure 1) are known. However, these values differ in different lasers. Therefore, modifications to the circuit shown in Figure 6 are shown in Figures 10-11 to compensate for these variations. In Figure 10 the resistor 44 has been replaced with a variable resistor 44. Since this resistor 44 sets the value of the time constant of the RC circuit to be equal to the time constant t fs of the circuit, changing this resistor provides adjustability in the RC circuit. The time constant of the RC circuit can be adjusted by varying the value of the resistor 44 to match the time constant t fs of the laser. This adjustment of resistor 44 is equivalent to adjusting the "R" term in the exponent of the equation I = Ae (-t/RC) . In Figure 11, a variable resistor 50, has been connected to the two pole switch S between the two pole switch S and the resistor 44 at node 45. The other end of the resistor 50 is shown connected to a voltage source 52. The resistor 50 affects the offset adjustment of the thermal effects correction signal 26. By changing the value of the resistor 50 the offset adjustment can be set. Resistor 51 has been added between to receive laser control signal 36 prior to two pole switch S to insure correct operation of resistor 50 to provide a voltage change at node 45 when it is adjusted. Offset adjustment is used since laser output is not directly proportional to laser input current. The offset adjustment then adds an appropriate amount of current to achieve the desired laser output. The addition of resistor 50 is equivalent to adding a new term "B" to the current equation to produce a new equation I = Ae (-t/RC) + B where B = an offset voltage. In Figure 12, an additional variable resistor 54 is shown connected between the thermal effects correction signal 26 on line 28 and the junction between resistor 44 and capacitor 46. Variable resistor 54 is bidirectionally connected to the adder 34 through the thermal effects correction signal 26 on line 28. The other end of the variable resistor 54 is connected to ground 56. The resistor 54 affects the scaling factor used to create the thermal effects correction signal 26. By changing the value of the resistor 50 the scaling factor can be adjusted. The adjustment of variable resistor 54 is equivalent to adjusting the "A" term in the equation I = Ae (-t/RC) . This circuit has been greatly simplified for clarity of explanation. It must be remembered that standard design techniques should be used when designing the actual circuit. For example, the two pole switch S may be implemented using a multiplier, and outputs should be adequately buffered to avoid an undue load on individual points in the circuit. Thus far, circuitry to compensate for self-heating effects have been shown. In Figure 13 a circuit to compensate for cross heating effects is shown. The thermal effects simulators 24, 64 of two adjacent laser systems are bidirectionally connected to a third cross thermal effects simulator 82, designed to compensate for cross thermal effects, through the thermal effects correction signals 26, 66. Each laser system is comprised of a video data input signal 20, 60 which is fed to its respective thermal effects simulator 24, 64 on line 22, 62 and a laser and driver assembly 40,80. The thermal effects simulators 24, 64 also receive input from its respective laser control signals 36, 76 on lines 38, 78. The thermal effects simulators 24, 64 then produce thermal effects correction signals 26, 66 on lines 28, 68 which is sent to adders 34, 74 to be combined with automatic gain control signals 30, 70 in adders 34,74. The adders 34,74 produce laser control signals 36, 76 which controls the lasers in the laser and driver assemblies 40, 80, respectively. The resultant correction current will have the same shape as the curve shown in Figure 8. Figure 14 illustrates a simple version of the circuit inside the thermal effects simulator 82. Two series resistors 84, 86 are connected to the bidirectional outputs of the thermal effects simulator 82. A capacitor 88 is connected to ground 90 on one end and between the two series resistors 84, 86 on the other end. Resistor 86 is used to set a time constant for one assembly and resistor 84 is used to set a time constant for the other assembly. Since both resistors 84, 86 are variable resistors then the time constants can be adjusted by varying the resistance. Figure 15 shows a variation of Figure 14. Resistor 92 is added between capacitor 88 and the junction of resistor 86 and resistor 84 to provide better tunability of time constants. Again, there are many ways, known in the art, to create the thermal effects correction signal 26 shown in Figure 14. The RC circuit could be replaced with an equivalent LC circuit, as known in the art, or the signal could be approximated digitally, using a miroprocessor, ROM, RAM, or other digital circuit, as known in the art.

Claims: A laser with intensity control comprising: a) a laser (40), b) simulator means (24) for simulating laser thermal effects and generating a compensating signal (26) in accordance with the simulated laser thermal effects, the simulator comprising: i) a control means (S) for receiving a control signal; ii) an input means (38) for receiving an input signal (36); iii) a current generating means (28, 44, 46; 50; 52; 54) for generating said compensating signal said current generating means being operably connected to said input means and to said control means, and being responsive to said control means to generate said compensation signal only when said control means has received the predetermined control signal, said current generating means being operably connected to said control means to generate the increasing current compensating signal when said control means has received the control signal; and iv) output means (28) operably connected to and responsive to said current generating means for transmitting the compensating signal, v) said compensating signal being approximately described by the equation I=Ae (-t/RC) +B wherein: A = a first constant, and B = a second constant corresponding to an offset voltage, e = the base for natural logarithms approximately equal to 2.71828, and the elements in the exponent: t = time constant of the circuit, R = resistance of the circuit, and C = capacitance of the circuit, c) means providing a base level gain control signal (30) for driving said laser, d) adding means (34) operatively connected for receiving the base level gain control signal (30) and the compensating signal (26), for combining said base level gain control signal and the compensating signal to generate a laser driving signal (36) for controlling the intensity of the laser, and e) said laser (40) being operatively connected for receiving the laser driving signal. The device of Claim 1 wherein said current generating means (28, 44, 46; 50; 52; 54) is a circuit comprising at least: a) a first resistor (44) with first end and a second end, b) a capacitor (46) with first end and a second end, c) said first resistor being arranged with its first end electrically connected to said control means and its second end electrically connected to the first end of the capacitor, d) said capacitor being connected to ground at its second end, and e) said output means (28) being connected at a junction of the resistor with the capacitor. The device of Claim 1 wherein said current generating means (28, 44, 46; 50; 52; 54) is a circuit comprising at least: a) a resistor and an inductor; b) said resistor and said inductor being so constructed and arranged to generate the compensation signal approximately described by the equation I=Ae (-t/RC) +B. The device of any of Claims 1 to 2 wherein said current generating means (28, 44 46: 50; 52; 54) is a microprocessor, RAM or ROM so constructed and arranged to generate the increasing current compensating signal approximately described by the equation I=Ae (-t/RC) +B. The device of Claims 2 wherein said current generating means (28, 44, 46; 50; 52; 54) includes: a) a second resistor (50) electrically connected on one end at the same connection that said first resistor (44) is connected to said control means (5); and b) a voltage source input means (52) electrically connected on the other end of said second resistor. The device of Claim 5 wherein said current generating means (28, 44, 46; 50; 52; 54) includes a third resistor (54) electrically connected on one end at the same connection that said first resistor is connected to said capacitor (46) and electrically connected on the other end to said output means (28). The device of any of the preceding Claims 2, 5 or 6 wherein said first resistor (44), second resistor (50) and/or said third resistor (54) is a variable resistor. The laser of any of the preceding Claims wherein said base level gain control signal (30) is fed to said simulator input means, or said laser driving signal (36) is fed to said simulator input means. A laser system with intensity control comprising: a) first and second lasers (40, 80), b) first (24) and second (64) simulator means for respectively simulating first and second laser thermal effects and generating first (26) and second (66) compensating signals respectively in accordance with the first and second simulated laser thermal effects, c) first (30) and second (70) base level gain control signals for respectively driving said first and second lasers, d) first (34) and second (74) adding means operatively connected respectively to said first and second base level gain control signals and the first and second compensating signals for respectively combining said first and second base level gain control signals and the first and second compensating signals to respectively generate first (36) and second (76) laser driving signals for respectively controlling the intensity of the first and second lasers, e) said first and second lasers being respectively operatively connected for receiving the first and second laser driving signals, f) a third simulator means (82) for simulating cross laser thermal effects and generating a cross laser compensating signal in accordance with the cross laser simulated thermal effects, and g) said third simulator means being operatively connected to both said first and second adding means. The laser system with intensity control of claim 9, wherein said third simulator means (82) comprises: a) a first resistor (86) with a first end and a second end, b) a second resistor (84) with a first end and a second end, c) a capacitor (88) with a first and a second end, d) the first end of said first resistor being operably connected to the first end of said resistor and the first end of said capacitor, e) the second end of said first resistor (86) being operably connected to said first adding means (34), f) the second end of said second resistor (84) being operably connected to said second adding means (74), and g) the second end of said capacitor (88) being operably connected to ground (90). The laser system with intensity control of claim 9, wherein said third simulator means (82) comprises: a) a first resistor (86) with a first end and a second end, b) a second resistor (84) with a first end and a second end, c) a third resistor (92) with a first end and a second end, d) a capacitor (88) with a first end and a second end, e) the first end of said first resistor being operably connected to the first end of said second resistor and the first end of said third resistor, f) the second end of said first resistor (86) being operably connected to said first adding means (34), g) the second end of said second resistor (84) being operably connected to said second adding means (74), h) the second end of said third resistor (92) being operatively connected to the first end of said capacitor (88), and i) the second end of said capacitor (88) being operably connected to ground (90).

Inventors: BUHLER STEVEN A, HARRIS ELLIS D, BUHLER, STEVEN A., HARRIS, ELLIS D.
Applicants: XEROX CORP, XEROX CORPORATION
Application year: 1994
Main classification: H01S3
IPC codes: H01S5